Digital computers far exceed the capabilities and speeds of analog machines for complex mathematical processing. However, a large amount of information that may be processed by digital machines exists in analog form. The analog signals must be converted to digital form before they can be subjected to digital analysis, manipulation and storage.
Electronic conversion of analog signals to digital format is an established technology. But as digital processing capacity and speed has increased with the evolution of better storage and computation hardware, there has been increasing demand for faster analog to digital (A/D) converters to provide higher sampling rates and allow conversion of ever higher frequency analog signals to digital form. Electronic A/D conversion has a frequency limit at this writing of about 6 GHz due to thermal dissipation and fundamental speed limitations of electronic circuits that respond to signal input and create digital output.
High speed electronic A/D conversion requires complex circuits. Typical N-bit flash A/D converters require 2.sup.N -1 comparator circuits whose outputs must be encoded to produce an N-bit parallel output. Thus, an eight-bit flash converter requires 255 comparators and an eight-bit encoder device to produce an eight-bit parallel output. The electronic encoding of the comparator output is also a major factor in the fundamental speed limitation of electronic converters.
The development of optical analog and data transmission techniques has enabled the expansion of signal bandwidths and operation into higher and higher frequencies for many kinds of systems. These systems must be interfaced with digital computers to take full advantage of the signal processing capabilities of the digital systems. Therefore, it becomes necessary to develop even faster A/D converters to sample the extremely high frequency signals carried on and by lightwaves. Because speed is related to circuit complexity, the simpler the circuit is the faster it may perform. The present invention is directed to providing an A/D converter for lightwave signals, capable of operation into frequencies far above 6 GHz, that is less complex than an electronic A/D converter on a per bit of output basis.